This invention relates to seals and seal assemblies, often referred to as packoffs and packoff assemblies, respectively, for sealing the annular space, often referred to as an annulus between a wellhead casing hanger and the surrounding cylindrical wall of a wellhead. This invention is specifically directed to improving such packoff and packoff assemblies so that packoff may be retrieved from the annulus after having been set, i.e., placed in position of sealing engagement between the casing hanger wall and the surrounding cylindrical wall.
For an example of a type of packoff and packoff assembly to which this invention relates, attention is directed to the U.S. Pat. No. 3,797,864 of Hynes and Ortolon, and to U.S. Pat. Application of Slyker and Pettit Ser. No. 419,270, filed Sept. 17, 1982, now U.S. Pat. No. 4,521,040. These patents illustrate a packoff comprising an elastomeric ring interposed between two metallic seal rings. Each metallic seal ring has a pair of lips extending toward the elastomeric material so that on compression of the elastomeric material the lips are forced outwardly toward the surrounding walls.
It is recognized that a packoff of the type disclosed in these patents perform as anticipated when set, but often their retrieval is required. To retrieve such packoffs, the packoff assembly is pulled out of the annulus, i.e., pulled upwardly back out of the well by pulling on the upper metallic seal ring. This pulling often resulted in a rupturing or tearing of the elastomeric material leaving half, or some part thereof, together with the lower metallic seal ring within the annulus. When this happens, when the remaining parts of the packoff cannot easily be retrieved, other sealing arrangements must be made.
Recognizing also that the use of the elastomeric type seal is the best approach to packoff off the annulus, numerous attempts have been made in the past to provide a means by which such a packoff can be retrieved.
One such attempt comprised a plurality of vertically disposed bolts extending through the upper and lower metallic seal rings and through the vertical center portion of the elastomeric material with sufficient play between the lower seal ring and the bolts to allow the bolts to move axially relative to the lower metallic seal ring when the packoff was set in the annulus. This allowed the elastomeric material to deform and allowed the distance between the metallic seal rings to change. When the packoff was set, however, it was found that, in the areas immediately surrounding the bolts, the elastomeric material differed in its deformation than in the areas between the bolts. This resulted in different sealing capabilities near the areas of the bolts from that of the other areas. Thus, while this prior art attempt solved the problem of retrievability, the sealing capability of the packoff was compromised, i.e., the sealing capabilities around the packoff varied circumferentially around the packoff and provided leak paths.
Another attempt to make such a packoff retrievable was to use wires instead of bolts, but the result was the same.
Another approach to providing a packoff with retrievable capabilities is the use of a solid, cylindrical band disposed vertically midway of the elastomeric material and connected in some suitable manner between an upper and a lower metallic seal ring so that a pull on the upper metallic ring, pulled the lower metallic ring without stress on the elastomeric material. This approach divided the elastomeric material into two separate elastomeric seals: one adjacent the casing hanger and one adjacent the surrounding well housing wall. In this approach, in addition to having variable elastomeric distortion since the elastomeric material was no longer a single body of material, the inner solid metallic ring provided four possible leak paths instead of two in the annulus one leak path adjacent surrounding the wellhead wall, one on each side of the metallic band and one adjacent the sealing wall of the casing hanger. Again, while the solid band approach solved the retrievable problem, it compromised the capabilities of the packoff to seal the annulus.
Still another approach for providing a packoff with retrievable capabilities is shown in the U.S. Pat. No. 4,324,422, of Rains and Reimert in which a pair of metallic rings with dog-like portions, i.e., overlapping fingers, were imbedded in the elastomeric material and arranged in a lost motion connection between the fingers. The lost motion connection allowed deformation of the elastomeric material when the packoff was set and became interlocking when the packoff was to be retrieved by providing a mechanical connection between the upper and lower metallic seal rings when the fingers engaged one another in an upward pull on the upper seal ring. This approach, however, while again solving the retrieval problem, compromised the sealing capability of the packoff. The metallic sleeves and fingers, together with the lost motion connection, interfered with the deformation characteristics of the elastomeric material. Stated another way, the elastomeric material did not deform as a single solid body of elastomeric material would have deformed, when set.
Another type of packoff which is part of the prior art because it has retrievable capabilities is one utilizing upper and lower metallic rings with a deformable metallic band therebetween and surrounded by a plurality of elastomeric rings. The deformable metallic band is in the form of a plurality of V's laid sidewards having the elastomeric rings at the apex of the V's. When set, the ends of the legs and the apices of the V's and, together with the elastomeric rings, engage the annulus side walls to form a seal. This packoff, however, relies heavily on the metal-to-metal sealing engagement against the annulus walls. The use of several elastomeric rings, instead of one body of elastomeric material with different deformation characteristics, may provide several leakage paths. Nonetheless, a pull on the upper metallic ring will retrieve the packoff since the deformable metallic band is connected to the lower metallic ring. Other disadvantages in this type of packoff are the severe difficulties in coping with even small eccentricities between the casing hanger and wellhead housing. This type of seal also needs a very high amount of force to set, which is supplied by undesirable torque multiplication gears.
Another packoff which has retrievable capabilities is that shown in the U.S. Pat. No. 3,561,527 of Nelson which an elastomeric ring between two upper and lower seal rings included a centrally located, vertically oriented serpentine shaped band imbedded in the elastomeric material. The connection of the serpentine band to both the upper and lower seal rings made the packoff retrievable, but in view of the size and thickness of the serpentine band, like the packoff of the U.S. Pat. No. 4,324,422 of Rains and Reimert, supra, the metallic serpentine band interfered with the deformation characteristics of the elastomeric material and reduced the sealing capabilities of the packoff.
It is therefore an object of this invention to provide a packoff similar to the prior art utilizing two metallic seal rings with a ring of elastomeric material therebetween, but with retrieval capabilities which will not interfere or compromise the sealing capabilities of the elastomeric material.